Adjacent channel selectivity is a measurement of a radio receiver's ability to receive a desired signal on a first channel in the presence of a much stronger signal on an adjacent, second channel. To perform this measurement, two signal generators are used: a first generator tuned to the desired channel and modulated with a first tone, and a second generator tuned to the adjacent channel and modulated with a second tone.
In an exemplary measurement methodology, the first signal generator's output level is adjusted to provide a specified SINAD measurement in the receiver under test (typically 12 dB). The second generator's output level is then increased until this SINAD measurement is degraded a specified amount (typically 6 dB). The output signal levels of the two generators are then compared and their ratio converted into logarithmic form to yield an adjacent channel selectivity measurement expressed in decibels.
A related procedure is termed "compliance testing" and involves setting the amplitude of the in-and out-of channel signals at known, fixed values and confirming that the resulting SINAD measurement is below a given value.
The adjacent channel selectivity measurement procedure has a number of drawbacks that, for years, have been suffered rather than cured. A principal drawback is that the measurement accuracy depends on the accuracy with which the amplitudes of the signals produced by the two generators can be determined. These amplitudes are typically determined by reference to calibrations on RF step attenuators used in each generator to attenuate its nominal output level down to the desired value. While attenuators can be well characterized at audio frequencies, radio frequency attenuators unavoidably have non-flat frequency responses due to stray reactances. The inaccuracies of such attenuators become particularly acute at high frequencies. (Some signal generators span frequencies extending into the gigahertz range.) Thus, it is difficult to accurately ascertain the ratio of amplitudes of the two signals used in making the adjacent channel selectivity measurement.
A related problem is that of matching the nominal output level of the two generators. The nominal output level (the level from which the attenuators attenuate) is set in each generator by automatic level control circuitry. While such circuitry is effective to eliminate variations in levels over time, there is typically no provision for sharing a reference level between two generators so that both outputs can be set to precisely the same level. Absent a shared reference level, component tolerances in the reference circuits introduce additional uncertainties into the adjacent channel selectivity measurement.
An additional drawback to the prior art measurement procedure is the requirement for two signal generators.
In accordance with the present invention, these drawbacks of the prior art are overcome by generating the desired channel signal and the adjacent channel signal using a single signal generator. The output from this generator is modulated with a signal from an offset oscillator to produce a carrier and a sideband signal, one in each channel. The ratio of the carrier amplitude to the sideband amplitude is set by the system's modulation index, which is controlled by an audio attenuator on the output of the offset oscillator.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.